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Triumph Repair/TR6 Mixture Adjustment Balance


Hello Howard:

I recently replaced the plugs in my 1976 TR6.  I noticed that the front 1 -3 plugs I pulled had blackened electrodes while the 4-6 plug electrodes were more brownish in color.  I think this might indicate that the front carb is running richer than the rear??  I synched the carbs with a Unisyn and the car runs fine with the new plugs but I think I need to address the mixture issue to make sure both carbs are set the same.  Do you have a write-up of the proper procedure to adjust and test the mixture?  

Also, I'm not sure what to do with the idle trim screws on the front of each carb.  They were wide open when I checked them and I have shut them down to 1 turn.  Some blogs suggest they should be closed completely.

Fyi, the emission system has been removed from this vehicle if that makes any difference in the set-up.



Hi John,

Sorry for the delay in answering, my PC has been down for a few weeks.

The Stromberg carburetors mixture adjustments are in the top down in the bottom of the oil reservoir with a 3mm Allen wrench. But that is not all that can make the front carburetor be overly rich.

It could be flooding from a float or float needle and seat problem.
It can also be a choke problem.

Balancing the throttle plates with the Unisyn gauge only syncronizes the throttle plates and has no effect on richness.

If you are talking about the large brass trim screws in the sides of the carburetors. They are not used at all and should be turned all the way in tight. They were only used by the factory when testing and each dealership was suppose to turn them all the way in.

If you have the manual chokes first check that they are not held open by the cables. If you have the coolant operated chokes, You should disable them by loosening the mounting screws that hold them to the sides of the carburetors and slip a piece of tinfoil between them and the side of the carburetors and snug the screws up to disable them. If you have the auto chokes that have the plastic cover on the side with a single nut in the center and have a bi-metal spring inside then you need to remove the small screws holding the retaining ring and you will find a lever inside and you can then manually move the lever to close the choke off for testing.

You should also remove the top pot on the front carburetor and examine the rubber diaphragm for any holes as that too can cause a extra rich mixture on the front carburetor and the flat black  carbon on the front plugs is a sure indication of a rich mixture.

Let me know,


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Howard M. Fitzcharles III


Triumph TR-4 up & Spitfire, and Engine theory


Dealership line mechanic on MG, Triumph, Jaguar for 15 years, Instructor in commercial mechanics school 2 yr. Product information manager for piston and valve manufacture, Instructor & hotline answer man for import car parts importer 15 yrs.

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